The basic answer is Yes, a competent machine shop can port and polish the head. However, If your engine is completely stock then there are quite a few things you should do to it before getting it ported and polished. You need to improve flow in other areas as well. New intake such as a K&N filter and Ram Air and possibly larger throttle body(matched to intake manifold). A larger exhaust system 2.25 or 2.5" with a free flowing catalytic converter. THEN, when you have the engine apart for a port and polish you should replace the cam with higher lift longer duration, also replace the timing chain and sprockets and do a valve job too. THEN, you will see results of a port and polish.
I'd pass on it unless they have all the tools for doing a good job and have references you can check that are reliable. This includes flow testers and such and if it's just some guy with a set of hand tools I'd def. stay away. You'll end up with a head that is worthless. The most I'd ever do to a Z head is port match to the manifold, you won't see much better use out of a street car anyway, porting is only going to make it better for high RPM use anyway and has to be done to the type of cam you are going to use. If you have some books like how to hot rod datsuns they have a pretty good bit about porting and they say there isn't many shops set up to properly do this and even some that are are only as good as the person doing it even with all the best epuipment, to me that is one gamble I would avoid.
The Motor legend Smokey Yunick had a great way of porting in and outlets.
Since NASCAR rules checked the ports by its surface roughness (porting was totally prohibited) he adapted ahose to the port, ran water blended with carborundumunder high pressure through it.
The ports became big as caves but went through the inspection as well!
Of course this way needs proper tooling as well,all ways are good, except the bad ones.
Good Luck with your tuning
Marten Carlsson Sweden